The portable power industry has traditionally been using charge rates between 0.7C and 1C (“constant current” charge rates) when charging electronic devices, which is the rate used for laptop computers. This current allows the notebook computer's battery pack to be charged at constant currents that are 70% to 100% of the value of rated capacity of the cells. For example, in a battery pack containing 18650 cells, rated at 2.2Ah, in a 2p3s configuration (two cells in parallel, three cells in series), a charging current of 1C would be equivalent to a constant charging current of 4.4 A for the pack. This charging current is allowed until a maximum voltage (Vmax) is reached, which is typically set at about 4.2V. Once Vmax has been reached, the current is lowered by control circuitry to disallow, in this example, any of the three blocks of two parallel cells to reach voltage levels higher than 4.2V. In addition to the current being limited, the charging rate is even slower once Vmax has been reached. Electronic circuits managing this type of functionality are known in the art and have been implemented in battery packs for notebook computers. For a notebook computer, typical charging times are of several hours to reach a fully charged battery.
Safety and battery life are the main problems with providing faster charging. Practically, for lithium ion (Li-ion) batteries during fast charging, batteries may locally display overcharging, which may deposit lithium onto the carbon anode. This lithium deposit lowers safety of the battery, which may more easily go into thermal runaway, increase its internal gas pressure, and eventually explode. Another problem with fast charging is the rapid change of electrode dimensions, such as thickness variation. Mechanical degradation of the electrode structure is faster during this relatively fast charge than what would be the case for slower charging. These limiting features concern all Li-ion batteries, more or less, depending on battery design. Batteries may be designed to take charge faster by limiting impact of detrimental aspects, such as safety and battery life.
However, for batteries having multiple cells in parallel, a particular concern arises when trying to quickly charge battery packs. This concern has to do with the imbalance of cells in parallel. Impedance and capacity degradation is different between cells due to differences between cells during manufacturing and environmental exposure after manufacturing (i.e., temperature, vibration, mechanical shock, etc.). This means that two cells, having initially similar conditions in terms of (i.e., capacity and impedance), will display different performance after a few months of use. Each block of parallel cells will be limited by the weakest cell, having lowest capacitance and/or highest impedance, as this is the cell that will reach Vmax earlier than the cell having better characteristics. As cycling progresses, the weakest cell will degrade even quicker, as it will always be the cell that experiences the most extreme conditions. Safety is also a concern as performance is decreased. The cell having the lowest performance will normally be the cell having the highest chance of being overcharged, thereby being a safety concern.